1. Field of the Invention
This invention relates in general to electronic circuits and more specifically to negative voltage generation in electronic circuits.
2. Description of the Related Art
Negative voltages are utilized in electronic circuits. For example, a negative voltage is utilized in a programmable memory for erasing non volatile memory cells. Generating negative voltages for electronic circuits may present challenges for circuit designers in that the gate oxide of devices used to generate the negative voltages are susceptible to being over stressed.
FIG. 1 is a circuit diagram of a prior art level shifter. Level shifter 101 includes inverters 103, 107, 111, and 116, NAND gates 105, 109, and 114, P-channel transistors 115, 117, 119, and 121, and N-channel transistors 123 and 125. P-channel transistors 115, 117, 119, and 121, and N-channel transistors 123 and 125 form a latch circuit 113.
In operation, level shifter 101 takes a voltage at its input (IN) and latches the same logic state at its output (OUT). In one example, the IN signal has a voltage signal range of 0 to 3.3V and OUT signal has a voltage signal range of Vneg to 3.3 Volts. After the value is latched, the Vneg voltage is lowered from 0 volts to a negative value (e.g. −4 Volts) while maintaining the logic state of the output. After the voltage level has been lowered, a protection mode (PM) signal is asserted which causes the output of inverters 111 and 116 to both be driven to the voltage level of their low voltage rails (e.g. 0 Volts). With the assertion of the PM signal, the “high” logic voltage level of the output (OUT) drops from 3.3 voltage to 0 Volts (the value of the low voltage negative rail of inverters 111 and 116) while still maintaining the logic state. Afterwards, Vneg is reduced to a lower voltage level. With this circuit, once Vneg is lowered, the output cannot change logic states.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The Figures are not necessarily drawn to scale.